Mobile Ionic Contamination
(MIC)
refers to the
presence of ionic contaminants in the active circuitries of
semiconductor devices, the most common of which are alkali ions
such as Na+, K+,
and
Cl-. It is
usually observed in gate oxide layers of MOS transistors. These
contaminant ions are free to move about, hence the phrase 'mobile ionic
contamination.'
This mobility is accelerated by
temperature
and
electric
field.
The mobile ions often enter
the gate oxide through the interface between the gate (usually metal or
polysilicon) and the gate oxide (usually SiO2).
Some of the ions then drift to the Si-SiO2 interface under
the influence of electric fields created by voltages applied to the
gate. Given the high mobility of these ions in SiO2,
they can drift under field assistance even at room temperature.
The presence of these ionic
contaminants at the gate-oxide and oxide-semiconductor interfaces and in
the oxide itself results in a mobile ionic charge,
Qm, which
can cause long-term changes in the threshold voltage, VT, of
the transistor. The VT
shift aggravates as more charges accumulate at the Si/SiO2
interface.
According to
S. Wolf and R. N. Tauber, a Qm value in the low 1010/cm2
range will cause a shift of only a few tenths of a volt for a MOS
device with a gate oxide of 1000 angstroms. However, a Qm value in
the 1010/cm2
range can result in VT shifts of
several volts. Thus,
reducing Qm density should be a key ingredient of any program designed
to eliminate MIC failures.
A high Qm
value can also promote the formation of conducting channels that
increase leakage currents. Gain reduction due to mobile ionic
contamination has likewise been observed. Bipolar devices can also
experience beta degradation due to the presence of mobile ionic
contaminants, mainly because these can change carrier concentrations.
Figure 1.
The mobile ionic contaminants present in the gate oxide can
accumulate as
an ionic charge that affects the channel of a MOS transistor.
Among the common
contaminants, Na+
exhibits the greatest mobility due to its small atomic radius. It
is also usually the first mobile ionic contaminant to suspect if MIC is
being dealt with because Na+
is widely distributed, being present in air and in human byproducts such
as perspiration and saliva.
<Proceed to Page 2 - MIC Reliability Issues / Failure
Analysis>
See
Also:
Die Failures;
Failure Analysis; Reliability Models
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